Soft glass having wide working range



Patented Oct. 31, 1950 SOFT GLASS HAVING WIDE WORKING RANGE William H.Armistead, Corning, N. Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York No Drawing. Application November 20, 1946,Serial No. 711,201

3 Claims.

This invention relates to compositions for glasses having specialcharacteristics fitting them for a variety of purposes such as themanufacture of tubes and bulbs and their subsequent fabrication intoelectric sign lights, cathode ray tubes and the like. Such purposesrequire a thermal expansion coefiicient in the neighborhood of 90 10"cm. per cm. per degree 0., a moderately low softening point, a wideworking range, a high electrical resistivity, good chemical and physicaldurability, low specific gravity and low batch cost for large tonnageproduction.

By working range is meant the temperature interval through which theglass is sufficiently plastic for working. The length of the workingrange is dependent upon the rate at which the viscosity of the glasschanges with temperature throughout this range and the more slowly theviscosity changes the longer will be the Working range. The rate ofchange of viscosity in the working range for most glasses issufliciently uniform so that the difierence between the temperatures fortwo given viscosities of a glass affords a convenient measure of suchrate for comparative purposes.

Two temperatures which may be determined for any glass by methods wellknown in the art and which have come to be standard in reference to thephysical properties of glasses are the softening point at which .thelogarithm of the viscosity in poises is approximately 7.65 and thestrain point at which the logarithm of the viscosity is approximately14.6. Otherwise defined, the softening point of a glass is thattemperature at which a thread of the glass .55 .75 millimeter i indiameter and 23 centimeters long will elongate under its own weight atthe rate of one millimeter per minute When heated throughout the upper-15 centimeters of its length. The strain point is that temperaturebelow which appreciable permanent strain cannot be established norremoved through plastic fiow, and it is measured preferably by themethod described by H. R. Lillie in the Journal of the American CeramicSociety, vol. 14, page 505 (1931) in an article entitled Viscosity ofGlass between the Strain Point and Melting Temperature.

A softening point between 610 and 720 C. and and a working temperaturerange of 215 C. or more are desirable in order to facilitate the flameworking of the glass and the sealing of metal parts thereto during itsfabrication into illuminating devices, cathode ray tubes, etc.

A thermal expansion coefiicient around 90 X 10- cm. per cm, per degreeC. has become a required property for glasses to be used for thesepurposes in order to match the expansion coefficients of the variousmetal alloys which have been developed for making the glass-to-metalseals.

For some purposes, such as the production of 2 cathode ray tubes, aglass having an electrical resistivity of at least 100x10 ohm cm. at 350C. is required.

Needless to say, the glass must be resistant to weathering and theattack of chemical reagents and to devitrification during flame Working.From the standpoint of economy, a low specific gravity and a low batchcost are essential.

Lead glasses or glasses of the type possess some of the above describedproperties and have heretofore been employed for the purposes set forth.However, as a result of the recent war, lead-containing batch materialshave become scarce and expensive and the need for glasses having thesame or better properties but containing little or no lead is verypressing. Lead glasses in general have objectionably high specificgravities and in this and other respects improvement is desired.

Glasses of the soda-lime-si1ica type also possess some of the desiredproperties but have too short a working range and are too low inelectrical resistivity, not over 2x10 ohm cm. at 350 C. for the presentpurposes.

Prior soda-a1umina-silica glasses in general are too hard or have toohigh softening points, have too low expansion coefiicients and are toolow in electrical resistivity.

Thus it will be seen that prior glasses, which have a sufiicient longworking range for the present purposes, do not at the same time possessall of the other necessary properties.

The object of this invention is to provide glasses which have all of theabove described desirable properties and which are particularly suitablefor the purposes set forth.

A specific object is to provide a glass having a working range greaterthan 215 C.; a softening point less than 720 C., an electricalresistivity greater than 2 10 ohm cm., an expansion coefiicient between80 10 and 10O 10 cm. per cm. per degree C., a specific gravity less than3, good chemical and physical stability and substantial freedom fromlead oxide.

I have found that the above and other objects may be attained with acomposition at least 94% of which consists of 53% to SiOz, 3% to 15%A1203, .1% to 13% K20, .1% to 17% NazO, .5% to 2% LizO, 0% to 28% BaO,and .5% to 2.5% F, the total percentage of. alkali metal oxides beingbetween 15% minus one-third the percentage of BaO and 20% minusone-third the percentage of BaO. Preferably, the glass contains at least3% BaO and the total percentage of alkali metal oxides is between 16minus A; the percentage of BaO and 19 minus the percentage of BaO. Inaddition to the above named constituents the glass may, if desired,contain up to 6% of other oxides, such as PbO' and oxides 2,527,693 V 34 of metals of the second periodic group other than that the newglasses, according to this test, are barium, without impairing itsproperties and generally better than these prior glasses. withoutseriously increasing its cost and specific Glasses 6 and 10 areparticularly suitable for gravity. the production of tubing forelectrical discharge A an important distinguishing feature of thedevices and glass 9 is particularly suitable for new glasses it is to benoted that they contain use in the manufacture of cathode ray tubes forlithia, fluorine and alumina. It is known that which a high electricalresistivity is required. both lithia and fluorine lower the softeningpoint As a cheap source of lithia, I have found it adof glass, butprevious experience indicates that vantageous to employ lepidolite inthe batches they also shorten its working range, that is, they of thenew glasses and it will be noted that their do not at the same timelower the strain point cost per pound of glass does not exceed about 36.

proportionatelywith the softening point. Alumi- In comparison, the batchcost of lead glasses is n-a, on the contrary, ordinarily raises thesoftenabout 35 to 5 per pound of glass. ing point but lengthens theworking range by not I claim:

raising the strain point proportionately. I have 1. A transparent glasswhich consists of 53% now found that in the new glasses the combinato75% S102, 3% to 15% A1 03, 0.1% to 13% K20, tion of lithia, fluorine andalumina not only low- 0.1% to 17% NazO, 0.5% to LizO, 3% to 28% 133.0ers the softening point but also unexpectedly and 0.5% to 2.5% F, thetotal percentage of the lowers the strain point proportionately with thealkali metal oxides being between 16% minus softening point. one-thirdthe percentage of BaO and 19% minus As an illustration but not alimitation of the one-third the percentage of BaO, the expansioninvention, the following compositions are given coeflicient of the glassbeing between 80 10' in percent by weight as calculated from the reand100' 10 cm. per cm. per degree C. the difspective batches on thecustomary oxide basis: ference between the strain point and the s0ften-III: III:

Some fluorine may be lost from the above ing point of the glass beingover 215 C., the softglasses during melting of their batches. Howeningpoint being less than 720 C. and the elecever, at least .5% of fluorinewill remain in the trical res t v ty being greater than 2X1 Ohm glassand may be determined by analysis. For 45 cm example, glass No. 6 aftermelting contained .94 2. A transparent glass which has the appart offluorine per 100 parts by weight of glass proximate composition inpercent by weight as determined by analysis. 66.5 S102, 7.5 A1203, 2.5K20, 11.5 NaQO, 1 LizO, 9

In the above table of examples it will be noted BaO and 2F. that in eachcase the working range as measured 3. A transparent glass consisting of53% to by the difference between the softening point and 75% S102, 3% to15% A1203, 0.1% to 13% K20, the strain point exceeds 226 C. In some in-0.1% to 17% NazO, 0.5% to 2% L120, up to 28% stances it may exceed 250C. or more. For some BaO, and 0.5% to 2.5% F, the total percentage ofpurposes, as for rapid mechanical production, it the alkali metal oxidesbeing between 15% minus may be desirable to restrain the working range55 one-third the percentage of BaO and 20% minus and to maintain itbelow its maximum. This may one-third the percentage of BaO, thesoftening be accomplished by the introduction of small point of theglass being less than 720 C., the dif amounts of certain oxides whichhave a shortenference between the strain point and the softening effect,such as MgO, CaO, ZnO, B203, and ing point being over 215 C., theexpansion co- T102. Not more than 6% of any one or a comefiie e t beingbe W e 30 10 nd X 0mbination of these oxides is required. The use of pDer deg and the electrical r8515- ZnO, CaO, and B203 for this purpose isillustrated tivity being greater than 2X 0 ohm cm. in compositions 4, 9and 10 respectively. WILLIAM H. ARMISTEAD.

In the table, the stability is expressed as the weight in milligrams persquare centimeter of the REFERENCES CITED Surf ce of the ss Which is1051? When a Sample The following references are of record in the of theglass is immersed in boiling fiftieth normal file of this patent: NazCOsfor six hours, and is determined by weighing the sample before and aftertreatment UNITED STATES PATENTS with the boiling NazCOz. The lower thisresult 7 Number Name Date the better the stability of the glass for thein- 2,392,314 Dalton Jan. 8, 1946 tended purposes. For comparison, itmay be said that the corresponding stability of common soda FOREIGNPATENTS lime glass is about .25 and that for the prior lead NumberCountry Date glasses is about .25 to .40. Thus it will be seen 7 870,982France 1942 Certificate of Correction Patent No. 2,527,693 October 31,1950 WILLIAM H. ARMISTEAD It is hereby certified that error appears inthe printed specification of the above numbered patent requiringcorrection as follows:

Column 2, line 31, for sufiicient read suyficz'ently; column 4, line 17for to Li O read to 2% L5 0 and that the said Letters Patent should beread as corrected above, so that the same may conform to the record ofthe case in the Patent Oflice.

Signed and sealed this 20th day of February, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

3. A TRANSPARENT GLASS CONSISTING OF 53% TO 75% SIO2, 3% TO 15% A12O3,0.1% TO 13% K2O, 0.1% TO 17% NA2O, 0.5% TO 2% LI2O, UP TO 28% BAO, AND0.5% TO 2.5% F. THE TOTAL PERCENTAGE OF THE ALKALI METAL OXIDES BEINGBETWEEN 15% MINUS ONE-THIRD THE PERCENTAGE OF BAO AND 20% MINUSONE-THIRD THE PERCENTAGE OF BAO, THE SOFTENING POINT OF THE GLASS BEINGLESS THAN 720*C., THE DIFFERENCE BETWEEN THE STRAIN POINT AND THESOFTENING POINT BEING OVER 215*C., THE EXPANSION COEFFICIENT BEINGBETWEEN 80X10-7 AND 100X10-7 CM. PER CM. PER DEGREE C., AND THEELECTRICAL RESISTIVITY BEING GREATER THAN 2X105 OHM CM.